Optical fibers are used extensively in telecommunications systems. In this document, the term “optical” is not limited to references to visible radiation but is used in a broad sense to refer to electromagnetic radiation of any wavelength or wavelengths that that may carry an information signal and that may be propagated along or through a waveguide device so as to deliver the information signal from one location to another location. It is often necessary to add or drop a wavelength channel in optical links or systems. Channel dropping can be achieved by a re-configurable channel dropping de-multiplexer. Such a re-configurable channel dropping de-multiplexer receives, from an input port, a wavelength division multiplexed composite optical signal that comprises several channels, or “wavelengths”. These plural channels include one or more “dropped channels” or wavelengths that a user wishes to remove, or “drop” to selected local channel drop ports, and, optionally, other “express” channels that the user wishes to send, as a wavelength division multiplexed composite optical signal, to another destination. The re-configurable channel dropping de-multiplexer permits the user to choose whether to drop or not to drop one or more pre-determined channels.
FIG. 7 illustrates a conventional single-dropped-channel re-configurable channel dropping de-multiplexer. The conventional single-dropped-channel re-configurable channel dropping de-multiplexer 700 comprises an optical input 702, a de-multiplexer 704 optically coupled to the optical input 702, a plurality of single-channel outputs 705.1, 705.2, . . . , 705.n optically coupled to the de-multiplexer 704, an optical switch 706 optically coupled to a first one 705.1 of the plurality of single-channel outputs, two switch outputs 707a–707b optically coupled to the optical switch 706, a multiplexer 708 optically coupled to the single-channel outputs 705.2, . . . , 705.n and to the first switch output 707a and an optical output 710 optically coupled to the multiplexer 708.
In the operation of the conventional single-dropped-channel re-configurable channel dropping de-multiplexer 700, a wavelength-division-multiplexed signal comprising the several wavelength channels λ1–λn is delivered from the optical input 702 to the de-multiplexer 704. The de-multiplexer 704 separates the channels so that each channel is directed to a respective one of the single-channel outputs 705.1, 705.2, . . . , 705.n. For instance, the channel λ1 is output to the first single-channel output 705.1, the channel λ2 is output to the second single-channel output 705.2, etc. Each one of the second through the nth single-channel outputs is optically coupled to the multiplexer 708 and delivers a respective optical channel to the multiplexer 708. The first single-channel output 705.1 delivers the channel λ1 to the input of the optical switch 706, whereat the channel λ1 may be directed to either one of the two switch outputs 707a–707b, depending upon the controlled operational state of the switch 706. If the channel λ1 is directed to the switch output 707b, then the channel λ1 is removed or “dropped” and the remaining channels λ2–λn are multiplexed by the multiplexer 708 so that the combined wavelength-division-multiplexed signal λ2–λn is output at the optical output 710. If, on the other hand, the channel λ1 is directed to the switch output 707a, then all of the channels λ1–λn are multiplexed by the multiplexer 708 so that the combined wavelength-division-multiplexed signal λ1–λn is output at the optical output 710.
Although the conventional single-dropped-channel re-configurable channel dropping de-multiplexer 700 can perform its intended function adequately, its use may lead to some disadvantages. A first disadvantage arises from the fact that all channels must be de-multiplexed and the re-multiplexed during operation of the conventional single-dropped-channel re-configurable channel dropping de-multiplexer, regardless of whether the channel λ1 is dropped or not. This causes un-necessary optical insertion loss for each channel as compared to an apparatus that does not perform the sequential de-multiplexing and multiplexing steps. Further, during this operation, each channel must pass through both a de-multiplexer and a multiplexer in sequence, the requirements for which add additional bulk, complexity and cost to the conventional apparatus as compared to an apparatus that does not perform the sequential de-multiplexing and multiplexing steps. Accordingly, there is a need in the art for an improved re-configurable channel dropping de-multiplexer. The improved re-configurable channel dropping de-multiplexer should be capable of delivering the non-dropped channels to the output without de-multiplexing and re-multiplexing these channels.